Modulation system



April 6, 1943. c. F. SHEAFFER 2,316,123

MODULATION SYSTEM Filed Sept 13, 1940 2 Sheeis-Sheet 1 Ummml m g 11 is L a i FROM AUDIO 2 r G g E g 8 AMPLIFIER D c o .9, u 3 31 IOOOOQ. g V .ooozs MF I Rd 1 85 41 43 I Li I OSCILLATOR BAL- Mop. MIXER TRIPLER POWER AMP.

MODULATOR smeas ca-5n Kcs. OUTPUT- OUTPUT- OUTPUT, OF 5.3 /3 Kc. TRANS MODULATION. 13735 Kw quzoo Kcs, MITTER CRYSTAL cRYs'rAL W 4:12 m u ,w

R .aaoo Kcs. OUTPUT 39 loosa x.

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C/zarles F Slim/f6]? Patented Apr. 6, 1943 L MODULATION SYSTEM Charles F. Sheafler, Tulsa, Okla., a ssignor to Radio Corporation of America Application September 13, 1940, Serial No. 356,689

11 Claims. (01. 179 -1715) My invention relates to modulation systems and, more particularly, to systems wherein a sig nal representing intelligence or other signal is impressed upon an outgoing carrier wave by causing the frequency thereof to depart more or less from a mean frequency determined by an oscillation generator. l

Itis an object of my invention to provide an. oscillator-modulator of the frequency shift type that'shall be extremely stable in operation. i Another object of my invention is to provide afrequency-shift modulation system that shall be substantially devoid of amplitude modulation effects.

Other objects will be apparent from the following description of a preferred embodiment of my "invention, when read in connection with the accompanying drawings wherein:

Fig. l is a block diagram. exemplifying a frequency shift modulation system comprising a preferred embodiment of my invention;

Fig. 2 is a schematic circuit diagram exemplifying an oscillator-modulator unit comprising a preferred embodiment of my invention},

Fig. 3 is a graph, wherein frequency is plotted against time of operation and indicating by curve a the frequency stability of my improved oscillator per se and by curve b the frequency stability of the oscillator-modulator unit; ;l"ig. ,4 is a plot of the equation, referred to hereinafter, indicating the theoretically attainable linearity, and, i t

Hg. 5 is a graph indicating the measured frequency modulation characteristics of the unit by thecurve a and after proper compensation thereof by the curve b.

, Before explaining my invention in detail, it will, be helpful to refer briefly to systems heretofore in use. There are three main types of frequency modulation transmitters, differing fromeach other in the specific method utilized toproduce the frequency-shift. (1) The Armstrong method described by D. L. Jaffe. in Proc. I. R, 15., April, 1938, wherein phaseshifts are produced by combining amplitude modulation side bands with a carrier of proper relative phase and the resulting frequency deviations are multiplied a number of, times to obtain the proper modulation band and outputfrequency; (2) the system recently developed by Robert E. Shelby, described in Electronics. February, 1940; and (3) systems utilizing the so-called reactance tube tube with a specially constructed target, and converts amplitude modulation into phase modulation. It is possible, with that system, to produce phase deviations many times greater than is possible by Armstrong's system. As a result,

fewer frequency. doubling stages are necessary and much better frequency stability is claimed.

A system of the third type was described by I. R. Weir in the General Electric Review, May and June 1940. It utilizes a reactance tube modulated oscillator of moderately high frequency and requires one doubler stage for deriving the proper modulation band and output frequency. The inherent instability of the oscillator frequency caused by the associated, reactance tube modulator makes'it imperative that some means of stabilizing the frequency be employed. Present practice is to beat down the frequency from some point in the circuit by means of a highly stable crystal controlled oscillator and to apply this derived frequency to an automatic frequency control device similar to the devices utilized for stabilizing the tuning of broadcast receivers; For transmitting purposes, it is highly important a that the A. F. C. circuit have a substantially linear output characteristic throughout the range of applied frequency deviation. If it does not have such a characteristic, variations in carrier frequency will result from theapplication of nonsinusoidal modulation potentials. This limitation dictates the minimum frequency to which the signal maybe beaten down and, therefore, limits the stability obtainable. r

Accordingly, another object of my invention .is to provide a system for producing frequency reactance tubes, wherein the effect of one of the reactance is so balanced against the effect of another reactance tube that in phase changes in the mutual conductance thereof will cause no appreciable change in the oscillator frequency.

Having in mind the defects of reactance tube systems heretofore known, my invention involves the use of an oscillator working at a relatively low base frequency and at least two reactance tube modulators associated with'the output circuit thereof and controlled in push-pull relation, so to speak, by the signal source. In addition, I utilize a combination of a balanced modulator,

-a mixer and a tripler, connected in cascade, by

means of which the output frommy improved lator tubes.

oscillator-modulator is raised to the radiation frequency without resorting to a large amount of frequency multiplication which, of course, would also multiply the slight amount of in stability inherent in the oscillator per se.

Inasmuch as one object of my invention is to dispense with A. F. C. circuits, another object ancillary thereto is the provision of a highly stablelow frequency oscillator.

Digressing somewhat, regardless of the method utilized to produce frequency modulation, the amount of control which can be brought to bear is directly proportional to the maximum susceptance that can be produced by the control device. Accordingly, any means which may be utilized to increase the ratio L's/E0, where Ix is the current through the controllable reactance and En is the voltage across theoscillator tuned circuit, will increase the frequency control ability of the modulator. Another object of my invention,

"therefore, is to provide an oscillator-modulator which would be adaptable to the use of small values of E and, at the same time, permit high values of Ix to be obtained. A related object of my invention is to provide an unit that comprises sufliciently low values of Lc/Co in the tuned circuit for maintaining good frequency stability, without too severely limiting the modulation capability.

Incidentally, in explaining the principles in-' volved herein, it is necessary to call attention to some of the inherent properties of the GSA! modu- This type of tube has two control .grids, the grid nearest the cathode, referred to in the claims as the excitation grid, controls the total emission and, hence, the current flow in both the screen grid and plate circuits, and another control grid, referred to in the claims as a modulation grid, which controls the current in the plate circuit, but has no substantial effect on the total emission current. It is a characteristic of this type tube for the screen grid to draw the major portion of the emission current. Since the voltage supplied to the excitation grid of each modulator is substantially constant in value, the emission currents and, hence, the automatic bias of each tube will also be constant, evenwith relatively high values of cathode biasing resistance, and since the modulation grids can cause no substantial change in the respective emission currents, the biases of the two modulators will remain constant during process of modulation. If a current change accidentally occurs, said change will cause a change in the cathode resistor voltage drop which will be applied to both grids simultaneously and which will be'oi' proper polarity to counteract the current change. The great amount of degenerative action exemplified in claim 21 is, therefore, the result of the use of relatively high values of cathode biasing resistance in the modulator tubes and due to the fact that a current change in a given tube feeds back a degenerative voltage into both its control grids.

Referring now to Fig. 2 of the drawings, my

preferred oscillator comprises two thermionic tubes, a 6SJ7 designated V1, and a 6F6, designated V2. The tube V1 supplies energy to a tuned output comprising an inductor I and a. tuning capacitor 3. Plate potential of about 6 to 110 volts is supplied to V1 from a 250 volt l C. source over a 50,000 ohm resistor 5 and a. choke I of approximately' 80 mh. inductance. The amplitude of oscillation, therefore, is approximately equal to the plate supply potential.

The grid of the 6F6 is fed directly from the tuned circuit, comprising condenser 3 and inductance l, and acts as a class A amplifier, and the amplified oscillations, inverted in phase, are fed back to the control grid of the 6SJ 7 and to the grid networks of the reactance tubes Vxl and Vx2 connected in shunt relation to the tuned oscillator circuit. The action of the reactance tubes will be explained later; however, it may be well to point. out that they exert their greatest effect when connected across the complete tuned circuit.

Each of the reactance tubes is of the 6SA'7 type, the space-current path in each tube being disposed in shunt relation to the tuned circuit of the oscillator. The reactance tubes are supplied with plate potential from the 250 volt source, a choke 9 of 30 mh. inductance being serially included in the supply circuit.

The control grids of the reactance tubes are supplied with modulating potentials in pushpull manner, from the secondary winding ll of an audio frequency transformer l3. For balancing purposes, the resistance element I5 of a potentiometer is connected across the secondary winding of the transformer and the movable contact I1 thereof is connected to a conductor l9 common to the cathodes of all of the tubes.

. It is possible, by making the assumptions usually permissible in an analysis, to set up and solve an equation for the frequency of oscillation of this circuit. This has been done by equating the voltages acting in it and solving for the frequency. To determine the effect of resistance that might be shunted across the tuned circuit, such a resistance is included in the equation and designated R. R0 takes into account the ratio of resistance to reactance, or Q of the coil, and R2 is the effective load resistance in the plate circuit of the 6F6 tube. The equation for the frequency is:

LULU

The conditions required for sustained oscillations are represented by the equation:

In any oscillation generation system, frequency instability may arise from either or both of two sources, Le, (a) variations in the tuned circuit values, caused by temperature and/or humidity changes and (b) variations in tube operating conditions, which result frorn supply voltage deviations, temperature changes, aging of tubes, etc.

Frequency drifts resulting from the first causecording to Equation 2 any admittance across the of six hours.

:Thecircuit constants indicated in Figure 2 of the drawings, though not critical, provide an oscillator having a high degree of frequencystability. Figure 3 is a graphical representation of the performance-of the circuit during a test run Curve a represents the performance of the'unit with'the reactance tubes inactive, il 3e., the performance of theoscillator per se. The actual stability i perhaps more truly reflected in the fact. that. 9.10% change in both filament and plate voltages gave rise to a frequency change of less than three cycles per secnd from the basefrequency of 350 kilocycles.

, A very important feature of my invention resides in so connecting the reactance tubes into the circuit that in-phase changes in the mutual conductance thereof, occasioned by any ex- ;traneous cause, will automatically offset each other and cancel out, thus producing no change i in the frequency of oscillations generated. Such a result I found possible for the reason that a reactance tube may be caused to introducethe effect of either capacity or inductance by proper selection of the phase of-' the grid excitation Referring once more to Fig. 2 of the drawings, it will be noted that the preferred embodiment of my invention comprises two reactance tubes, V11 and via, the first tube being for the purpose ofintroducing inductance and the second, capacity, into the frequency-determining tank circuit oftthe oscillator, as will be explained in more detail;

i It can be shown theoretically that if such a sys-' tem is so adjusted thatthe reactances, at the oscillation frequency, of the two reactance tubes and their associated networks are equal, changes in mutual conductance thereof that occur in phase will cause no changein the oscillator frequency, because one reactancetube will produce a reduction in tank circuit inductance and other will effect a proportionate increase in tank circuit capacity and vice versa. The net effect, of course, of the in-phase changes in mutual conductance is zero change in oscillation frequency.

going, yields:

The in-phase variations in mutual conductance tubes, temperature changes, etc., and they have substantially no effect uponthe frequency stability of the system. i

The general performance of the reactance tubes may be described as follows:

, The voltage across the plate load resistor 2| of the ,6F6 tube is, --NEo, N being the voltage amplii thatarise in actual operation are usually the resuit of changes in supply potentials, aging of theiication obtained from the 6F6. This voltage is applied to the phase shifting networks supplying thegrids of the two reactance tubes. The sa d networks, respectively, comprise a resistor 23 and an inductor 25 for thetube Vin, and a resistor "TI, and a capacitor 29 for the tube VXQ. The value of theresistance, R is exceedingly greater than [thereactance values of L; andC and, therefore,

:rthe networks cause phase shifts approximating 90degrees.

The voltage appliedto the grid of v11 will then JQL (R, N16 w I t the current in'each of the phase shifters and the plate current will be: 7

The reactance in shunt with the tuned circuit due to vxl will be:

work were in phase with the tank circuit voltage. action of the tube Vin, fore- An analysis of the N G012? C,R m Such tube, therefore, functions to introduce the effect of capacity, which is also inversely proportional to the square of the frequency.

I have derived a reasonably simple equation that expresses the frequency of oscillation, as-

suming that in the absence of a signal, sustained oscillations occur at the anti-resonant frequency resulting from the combined reactance of the tank circuit and the reactance tubes.

Such equation is as follows;

that the modulating potentials must ,be applied to the control circuits of the reactance tubes in a push-pull manner. The plot, therefore, is drawn from the points obtained by assuming that the two mutual conductances vary linearly. but in opposite directions with other.

Although the graph in Figure 3 is not exactly straight, it is a fair approximation, provided the frequency interval does not exceed plusor minus 50 kilocycles when utilizing an oscillator he I quency of 350 kilocycles.- The modulationchar acteristic maybe made substantially linear over the mentioned range by adjustment of the reactance tube grid biases in conjunction with pro portioning cf the modulation voltage suppliesfor the two tubes. For the latter purpose. the po.

tentiometer shunted across the secondary of the modulation transformer, is utilized.

Figure 5 shows graphically the result of two series of measurements made on an operating oscillator-modulator constructed according to, my invention. it Curve a was drawn by plotting the frequency deviations which resulted from the application ofsteps of voltage ofequal and opposite potential to the grids of the modulators and is similar to the curve obtained by plotting Equation 3.- Curve b was obtained by plotting maximum deviations vs. sine wave modulating voltage and, therefore, is an indication of the extent of linearity that may be obtained. f

Although I have mentioned 350kilocycl'esas The characteristics of the. above equation are respect to each the preferred oscillator frequency, it is not to be inferred that my invention is limited to that exact number. At frequencies lower than the preferred frequency, it progressively becomes more difllcult to obtain satisfactory modulation at higher modulation frequencies because of the resulting increase in tank circuit impedance at the modulation frequencies that occasions voltage fluctuations to be fed into the oscillator grid circuit, disturbing the operation thereof. There are methods by which this difllculty may be avoided, but in view of the more exacting side band filter requirements in the mixer output, they are not included herein.

The circuit adjustments necessary for making the frequency independent of the in-phase variations in mutual conductance of the reactance tubes may be obtained from Equation 3 by setting the ratio of the terms in the numerator equal to the ratio of the terms in the denominator. The required conditions then are:

of my invention because it minimizes amplitude modulation of the oscillator and effectively eliminates undesirable loading of the tuned circuit.

The plate circuit of the 6F6 tube in the oscillator includes a transformer 3| and a capacitor 33 connected in'shunt relation to the load resistor 2|. The inductance of the transformer 7 together with the adjustable capacitor provide means for adjusting'the phase of the voltage drop to the proper value. The load resistor 2| furnishes sufiicient damping to render negligible phase shifts resulting from frequency deviations within the modulation band. The output connection is made from the secondary winding of the transformer 3|.

It is necessary to thoroughly by-pass the 'cathode resistor in order to prevent it from acting as a frequency-modulation to amplitude-modulation converter, thereby producing unwanted amplitude modulation of the reactance tube grid supply. Such action on the part of the SP6 tube would result in accentuating any non-linearity of the frequency modulation characteristic. The circuit values in the drawings, if followed closely will give rise to an extremely stable oscillator-modulator; it is not to be inferred, however, that my invention cannot be practiced "through the utilization of circuit constants difiering from those shown.- The actual unit, under test, maintained its assigned frequency over a long period to within plus or minus 300 cycles. I

Referring now to Fig. 1, the manner in which the frequency modulated carrier wave may be obtained is exemplified by block diagrams. The output from my oscillator-modulator is mixed in a balanced modulator 35 with the output from a 3300 kc. crystal controlled oscillator 31. The upper sideband, which is 3650 kc. plus the modulation band of plus-minus 33% kc. is then selected and mixed with the output from a 10,083 kc. crystal controlled oscillator 39 in a conventional mixer 4|. Theupper sideband is again selected, thereby deriving a frequency of 13,733 kc. which is fed into a conventional tripler 43 which provides the carrier frequency and plus-minus 100 kc. modulation capability. A power amplifier 45 may be interposedbetween the tripler and the radiator.

If we assume that a stability of 10 cycles per megacycle can be obtained from the crystal controlled oscillators, andthat the modulated oscillator can be maintained to within plus-minus 300 cycles of its base frequency; an overall frequency drift of plus-minus 1130 cycles is possible, which is but A; of .01% at 41 megacycles. I have checked the system repeatedly over extended periods of time and the results indicate that a normal stability of better than .01% can be easily maintained at 41 megacycles, without the use of any sort of automatic means for continuously re-adjusting the oscillator frequency. One of my experimental runs is graphically shown by curve b in Fig. 3, which indicates that the frequency is substantially constant subsequent toa warm-up period of 15 minutes.

It is my personal belief, based upon numerous experiments and check runs, that the stability of my improved system is much better than that claimed for other systems of frequency modulation that utilize reactance tube modulation and automatic frequency control devices of the discriminator type. My system compares favorably, in both simplicity and stability, with the best of the other systems now in use. It is to be understood, nevertheless, that if' performance substantially that of a crystal controlled oscillator is necessary, an automatic frequency control device of conventional type may be utilized in conjunction with the oscillator-modulator, which may be so arranged that it will correct the oscillator frequency during intervals of low modulation.

Another advantage of my invention resides in the fact that substantially no amplitude modulation occurs and the stability is independent of the waveform of the modulating voltages.

Although I have illustrated and described a specific embodiment of my invention, modifications will be apparent to those skilled in the art to which it pertains. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. In a wave length modulation system, a wave generating means comprising a pair of electron discharge devices each having input and output electrodes, a tank circuit connected between the output electrodes of one of said devices, a coupling between said tank circuit and the input electrodes of the other of said devices, a coupling between the output electrodes of said other of said-devices and the input electrode of said one of said devices, a pair of electron discharge tubeseach having an anode, a cathode and a control grid, means coupling the impedan-ces between the anodes and cathodes of said pair of tubes in shunt to said tank circuit, a phase shifting circuit including a resistance and an inductance coupling the output electrodes of said other of said devices to the control grid of one of said tubes, aphase shifting circuit including a resistance and a condenser coupling the output electrodes of said other of said devices to the control grid of the other of said tubes, and means for modulating the impedances of said pair of tubes in push-pull relation in accordance with signals.

coupling the anodes and cathodes of said disthe phase of the voltages in said parallel arrangesource of,,oecillations,in1 dinaja tan' c cuit, a tube generator having an'anode electrode cou led we m st qns d t k ci u t nd a s idsq v db phase rsing means to said'po intpnsaidtank i in: a pl in ,rlativeiyidw direct current potentials to the'electrodes of said tube wherebyfoscillations of relatively lowvoltage are broached 'ands p said tank c rc t; mewsib! modulatingtheoscillations'generated comprbingj a pair ol reactarice devices each hav-g in: an anode and a cathode and a control: grid, 1 8 e e .thefi le i$ i l $ftln 1*! fi im e int. i fl i msihe w e h 'e e d'n i t' an? said -tank' circuit, f 'phaseffshifting @re'actances" coupled to the control electrodes ofsaidreactarice devices. conriecti nsbet een said phase shifting reactances and said' phase reversing means; means for modulatingthe jimpedancesgof said'pair oli vices l s -P eli'itid fi i o anc w t signals, and means, forv producing degeneration in-sa id devices.. a i

3, ,In a wave lengthlmodula'tion system; an elsetron discharge device oscillator having electrodes coupled with a tank circuit ior generating oscillations of relativelylow amplitude and substantially constant frequency, and means for modulating the a wave length of the oscillations generated comprising a pair of electron discharge tubes each having anode, a cathode and a control grid, means coupling the anodes of said tubes together and degrees appear on the anode and grid of each tube. and means for modulating the impedances of said tubes in phas opposition in accordance with signals.

4. In a wave length modulation system an electron discharge device oscillator having a tank circuit for generating oscillations of relatively low amplitude, and means for modulating the wavelength of theeosciilations generated including a pair of electron discharge tubes each having an anode, a cathode and a control grid, connections charge devices in shunt to said tank circuit said connections setting up on said anodes oscillations of a. first phase, an amplifier tube having input electrodes coupled to said tank circuit and excited by oscillations of said first phase, said amplifier tube having output electrodes, a variable con denser and an inductance in parallel coupled to said output electrodes of said amplifier, said parallel inductance and condenser serving to adjust merit, a phase shifter coupling the controlelectrode of each of said tubes to said parallel arrangement, and means for modulating the impedances of said tubes in phase opposition in accordance with signals. 70 5. In a wave length modulation system, a wave v generator comprising a pair of electron discharge devices each having input and output electrodes, a tank circuit coupled with the output electrodes of one of said devices and withthe input electrode other orjsaid devices=.serv1ng-;.to .reversezthaphase- 5 in accordance with signalsg wave length- 0f:whichds mabe' modulated flow ot the othersiofssaid idevices an output, cirgui comprising anfinductance, azcapacity esis ance effectively in parallel, coupled with the; put electrodes "of said other i or aiddevices, sai

of the- -oscillations in"said outputiriircuit -rcla, to the. phase 'of: the.oscillationsain-esaidgmmir cuit;- a pair ot reactance tubes-Thriving; inputoutput electrodes;meanscouplingthebutp e v trodes of -s'aid *reactancetubes .i-n shunt etossaid tank circuit} 'a' phase:shiftingareactancer coupling the -i'nput f electrodes offl each -:oi1+.-;said,-=reactance tubes to said-output circuit',:;and means fcrzmcdus, iatingtheimpedaneesthereo h eopposition 69in a'wave leii'gth'lmodulation system-1a tank-j circuit" wherein high frequency osc llations pair of' electron discharge: devices aeach r-haxing ana electron receivingelectrodesanielectrcnzzflow 9.0 4 trol" electrode and: {a cathodew mean-as, tyin t; electron receiving zelecti'odes-s togethersandjzt a: p'oint 'on said tankclrcuitg azmansaecilpling ha cathodes "of said 'devicesitoia'e second mointmnsa d tank circ'uit, a' phase I reversing? tuhewhavlng electrodes coupled-to said first pointslonisaid ankcircuit 'an'd -havingcutputo:electrodespz-a r-phase shitting reactance coupling the output electrodes of said phase reversing tube to the electron flow control electrode in each of said devices, said phase shifting reactances being of different character whereby the oscillations on the electron receiving electrodes and flow control electrodes of the devices are displaced by more than 90, and means for differentially modulating the impedances of said devices unequal amounts in accordance with signals.

7. A wave length modulation system including, a tank circuit wherein wave energy the wave frequency of which is to be controlled flows, a pair of electron discharge devices each having an anode, a cathode, and a control grid, 9. connec tion tying the anodes of said devices together and to a point on said tank circuit, a connection be-- tween the cathodes of said devices and a second point on said tank circuit, a wave amplifying and wave phase reversing tube having a control electrode coupled to said first point on said tank circult and having a cathode coupled to another point on said tank'circuit, said reversing'tube having output electrodes, a phase advancing network coupling the output electrodes of said reversing tube to the control grid of one of said first named devices, a phase retarding network coupling the output electrodes of said tube device to the control grid and cathode of the other of said first named tubes, the couplings being such that the alternating current voltages on the anode and control grid of each tube are displaced by more than 90 degrees, and means for modulating the impedances of said tubes difierentially in accordance with signals.

8. In a wave length modulation system, an oscillation generator including, a pair of electron discharge devices each having a control grid, an anode, and acathode, a tank circuit coupled between the anode and cathode of one of said devices, a coupling between the anode of saidone of said devices and the control grid of the other device and a feedback coupled between the anode of said other device and the control grid of said one device, a reactance tube having an anode, a cathode, and a control grid, a coupling between the anode of said reactance tube and the anode of said one of said devices, a coupling between the I cathode of said reactance tube and the cathode of said one or said devices, means for deriving from the anode of the other of said devices oscillations or the same phase as the oscillations on the control grid of said one of said devices, a phase shifting network coupling said last named means to thecontrol grid of said reactance tube, and means for modulating the impedanc of said reactance tube in accordance with signals.

v to produce regeneration in said tube and tank circuit, a reactance tube having an anode, a cathode. and a control grid, a substantially direct coupling between the anode of said reactance tube and the anode of said generator tube whereby the phases of the generated voltages on the anodes are the same, a phase shifting circuit coupled to the control arid and cathode of said reactance tube, a coupling for impressing said reversed volt age on said phase shifting circuit, and connections for controlling the impedance of said reactance tube in accordance with variable control potentials.

10. A system as recited in claim 9 wherein said phase reversing means also amplifies the generated oscillations fed to said phase shifting circuit.

11. In combination, an oscillation generator having a tank circuit for adjusting the frequency of the generated oscillations, at least two reactance tubes, each having a space-current path and electrodes including a control grid associated with said path, means coupling the space-current paths in said tubes efiectively in shunt to'said tank circuit, connections for impressing variable control phase of said oscillations in difierent directions and impressing said oscillations of. shifted phase simultaneously upon corresponding electrodes in said tubes, whereby frequency shift control may be attained and the resistance effect of the reactance tubes upon the tank circuit is minimized.

CHARLES F. SHEAFFER. 

